Recently, there have been remarkable technical innovations in machinery, component parts, and instruments, notably in the fields of electronics, atomic energy, and aerospace industries. These developments are whipping up widespread demand for the metals or alloy materials that were once regarded as very special. Today, such refractory metals as titanium, zirconium, hafnium, and vanadium and their alloys are in extensive use as quite common industrial materials. Indications are that metals with even higher melting points, e.g., niobium, molybdenum, tantalum, and tungsten, are about to play a prominent role as new industrial materials.
Ingots of the refractory metals or refractory metal-based alloy have thus far been made by either:
A) Compacting a refractory metal or its alloy in powder form under pressure and sintering the green compact to an ingot (sintering method) or; PA0 B) Compression-molding a refractory metal or its alloy in powder or sponge form or scrap of a refractory metal or its alloy into an electrode or, as an alternative, packing the material into a box or tube of the same material to provide an electrode, and then melting the electrode by the electron beam melting technique to form an ingot (electron beam melting method). PA0 a) Large contents of impurities (especially gaseous components such as oxygen, nitrogen, carbon compounds, sulfur compound, and hydrogen) in the resulting ingot place a limit upon its fabrication into high-purity products. This hinders the application of the method to the manufacture of members for high-temperature high-vacuum uses because of the possibility of objectionable gas release. PA0 b) Being a sintered material, the ingot poses a density problem. PA0 c) The process requires many steps and is costly. PA0 d) As a raw material for the ingot, scrap cannot be directly utilized. PA0 a) Pure titanium material PA0 The method comprises joining by welding blocks made by compression molding of pure sponge titanium that results from titanium purification process or lumps of pure titanium scrap or both to form electrodes for vacuum arc melting, melting the electrodes in a vacuum arc melting furnace, casting the melt into an ingot having a circular cross section, and forging it followed by rolling into a plate or bar product. PA0 b) Titanium alloy material PA0 The method comprises compression-molding pure sponge titanium and/or titanium alloy scrap with the addition of such alloy components as aluminum and vanadium, welding the molded articles together to form electrodes for vacuum arc melting, melting the electrodes in a vacuum arc melting furnace, casting the melt into an ingot having a circular cross section and forging it followed by rolling into a plate or bar product. PA0 a) Among metallic materials in use for structural members, there are some which have relatively high vapor pressures and are easy to obtain and process inexpensively, e.g., Ti, Fe, and Al. When a sheet, net, or the like of such a metal is used to envelop a virgin or scrap material of a refractory metal or a refractory metal-based alloy to be melted to form a meltable electrode and is melted with electron beams cold hearth remelting, impure gas components contained in small amounts in the material, such as O, N, S, C, and H, are effectively removed during the course of electron beam melting and surprisingly the enclosure component of a relatively high vapor pressure too can be preferentially evaporated from the molten material and depending on the melting conditions used (temperature, degree of vacuum, molten metal holding time, casting speed, etc.), the residual amount of the enclosure component can be controlled within a range from zero to a proper limit. Thus a component controlled ingot with an extremely small proportion of impurities such as gas components is obtained. This destroys the prevalent concept that the use of an enclosure of the same material as the charge for melting is essential for the preparation of an electrode to be melted. PA0 b) In this case, the enclosure material to be used is made from a metallic material easy to be lost by evaporation or a metallic material containing a component or components easy to be lost by evaporation accompanied by proper adjustments of the melting conditions, fine control of the alloy composition is permitted during electron beam melting, hence giving a refractory metal-based alloy with a desired composition in a stable operation. PA0 c) With titanium in particular, even when its meltable electrode is made by enveloping titanium sponge or titanium scrap with a sheet, net, or the like of aluminum or other metal having a higher vapor pressure than Ti or Ti alloys, good workability is assured as with a vacuum arc-melted ingot employed for the same purpose. An ingot with no contaminant from the enclosure may be produced. Although titanium sponge or titanium scrap is directly utilized as a material to be melted, the resulting slab is very sound with extremely low nonmetallic inclusions, such as LDI and HDI, and impurity elements, and with little compositional segregation. PA0 d) Furthermore, direct casting of the molten metal, melt refined by electron beam cold hearth remelting, to produce a square slab, and rolling without the need of forging in advance are now possible. These result in cost reduction with fewer process steps involved and render it possible to achieve an improvement in material yield due to the elimination of scalping which would otherwise accompany forging. PA0 1. a method of producing a refractory metal or refractory metal-based alloy material by electron beam cold hearth remelting which comprises melting and casting a meltable electrode, characterized in that the electrode used for electron beam cold hearth remelting is made by enveloping a material of refractory metal or refractory metal-based alloy to be melted with an enclosure formed from a metallic material having a higher vapor pressure than said particular refractory metal or from a metallic material which includes component or components having a higher vapor pressure than said particular refractory metal, PA0 2. a method according to 1 above wherein the material to be melted is a refractory metal-based alloy, the meltable electrode used for electron beam cold hearth remelting is made by enveloping the refractory metal-based alloy material to be melted with an enclosure formed from a metallic material having a higher vapor pressure than said particular refractory metal or from a metallic material includes component or components having a higher vapor pressure than said particular refractory metal, and the melting and casting of the electrode are carried out while adjusting the amount of evaporation of said higher vapor pressure material or component(s) during the melting, PA0 3. a method according to 2 above wherein the evaporation loss of the alloy component or components of the refractory metal-based alloy is compensated for with said metallic material or component(s) of the enclosure, PA0 4. a method according to 2 above wherein said metallic material or component(s) of the enclosure provides at least a partial addition of the alloy component or components of the refractory metal-based alloy, PA0 5. a method according to 2 above wherein a Mo-Ti-Zr alloy material is produced using a meltable electrode formed by enveloping Mo scrap which contains both Ti and Zr with a pure Ti enclosure, PA0 6. a method according to 1 above wherein the material to be melted is titanium sponge or titanium scrap or a mixture thereof and the meltable electrode is formed by enveloping a meltable material with an enclosure formed from a metallic material having a higher vapor pressure than titanium or from a metallic material includes component or components having a higher vapor pressure than titanium, the method comprising melting and casting the electrode to produce a slab with a square cross section, and then directly rolling the slab without subjecting the slab to forging before the rolling, and PA0 7. A method according to 6 above wherein titanium or a titanium alloy is made using a meltable electrode formed by enveloping titanium sponge, titanium scrap, or a mixture thereof with an enclosure of pure aluminum. PA0 a) An alloy of a refractory metal as the base and an alloy component metal having a higher vapor pressure than the base; PA0 b) An alloy of an alloy component metal having a higher vapor pressure than a refractory metal as the base and a metal having an even higher vapor pressure; PA0 c) An alloy of a refractory metal as the base, an alloy component metal having a higher vapor pressure than the base, and a metal having an even higher vapor pressure than the alloy component metal; or PA0 d) A mechanical composite of an alloy component metal having a higher vapor pressure than a refractory metal as the base and either a refractory metal as the base or a metal having an even higher vapor pressure than the alloy component metal or both.
These methods present problems for which there is a strong demand for solution.
Problems that have been pointed out in the practice of the sintering method include the following:
The conventional electron beam melting method, when used in producing an ingot of alloy based on refractory metal, entails much evaporation loss of the alloy components during melting, often resulting in an ingot with a composition outside the intended limits. Another problem is the high cost of making the electrode to be melted. It is due to the general belief in the art that the electrode must be manufactured by packing raw material into a box or tube of the same material as that to be produced to avoid the intrusion of foreign matter. Also in the case of compression molding, the process involves arduous, complex steps leading to high cost.
For these and other reasons, neither method has been deemed fully satisfactory.
Meanwhile, great strides have in recent years been made in the technology for the manufacture of especially titanium among refractory metals. There is a tendency, accordingly, toward a broader range of applications and growing demand for pure titanium and titanium alloys because of their excellent specific strength and resistance to heat and corrosive attacks.
Pure Ti and Ti alloy materials generally have been made by the following procedures:
These means conventionally employed for the manufacture of titanium materials, however, surface conditioning of the cast ingot or slab by frequent scalping at many stages during forging and rolling. This has offered the problem of low material yield and hindrance to cost reduction.
In addition to the high electrode cost, the ingot obtained by vacuum arc melting is prone to contain nonmetallic inclusions such as TiN and other low density inclusions (LDI) and WC and other high density inclusions (HDI). These inclusions cannot be disregarded, since they can cause cracking of the material, leading to deteriorated mechanical properties and shortened life of the final product.
In this connection attention is being paid to a new technology, electron beam cold hearth remelting as proposed in U.S. Pat. Nos. 4,681,627 and 4,750,542. The process consists of enveloping a metal or alloy ingot or material obtained by vacuum arc melting or the like with an enclosure of the same material as the ingot or material to form a meltable electrode 5 as shown in FIG. 1, and then remelting and purifying the same using an electron beam melting apparatus which comprises a melting chamber in which a water-cooled cold hearth 2 of copper is installed before a water-cooled copper crucible (mold) 1. The meltable electrode 5 is melted by electron beams 4 from electron beam guns 3, and the molten material is once held in the cold hearth 2 under a vacuum (a reduced pressure) to evaporate impurities from the melt for purification. At the same time, the molten metal is caused to overflow the cold hearth 2 and is cast semicontinuously into the water-cooled copper crucible 1 to produce a rod 6 having a circular cross section. It is a melting method claimed to be particularly suited for the melting and purification of refractory metals.
As regards the meltable electrode, the U.S. Pat. No. 4,681,627 defined in claim 1: "- - - charging the metal scrap into a tubular member with a closed end and another end, said tubular member being made of the same material as that of the scrap," thus indicating the use of an enclosure of the same material as the charge to be melted.
However, further improvements in the electron beam cold hearth remelting are sought, especially for the lower cost and higher quality of the product.